Delayed coking is a process by which a petroleum derived virgin or cracked residue or any mineral oil characterized by the predominant abundance of carbon and relatively minor amount of hydrogen atom within the compound's chemical structure, is rapidly heated in a prescribed manner at temperatures sufficient to start scission of such chemical bondage and therefrom fed to a vessel large enough to soak or reside for long periods of time. Such soaking in the so called coke drums effects the production of more valuable gases and lighter hydrocarbons, the hydrogen atoms of which are considerably increased at the expense of the co-product coke.
The gases and hydrocarbon liquid produced are analogous to those commonly seen in petroleum refineries, but the coke produced assumes two main classifications which are highly related to the aromaticity and quality of the feedstock residue. On the one hand, feedstocks that are less aromatic or more paraffinic produce coke whose crystallinic structure are fragmented and when calcined does not have a lustrous appearance. It is amorphous and with isotropic properties unsuitable for making high grade graphitized electrodes. Coke with more undesirable sulfur and metals in this class are normally used for fuel.
On the other hand, calcined premium coke produced from thermally cracked residues which are highly aromatic exhibits more crystalline structure, is characterized by its acicular or needle structure and with non-isotropic properties suitable for making high grade graphitized electrodes.
In the usual application of delayed coking the preheated stream of petroleum residue fresh feed is combined with the recycle stream produced from the fractionator bottom section and hence the combined charge is heated at prescribed controlled flow to the coker heater. The heater effluent at controlled temperature sufficient for coking reactions is fed to the coke drum, whereupon in a delayed manner, the heated mass is allowed to react further by thermal decomposition producing hydrocarbons, gases and coke. At the end of the usual 24 hours cycle, the heated mass is switched to the other coke drum for continuity of coking reaction. The previously coke filled drum is henced cooled at a prescribed schedule and decoked hydraulically in a manner well-known to those skilled in the art. The coke drum overhead vapors consisting of of the net product gases, hydrocarbons and the recycle stream enter the fractionator where various hydrocarbon products are separated and delivered to other downstream equipment for further separation and processing. The heavier recycle stream cut is separated from the heavy gas oil product via the process of reflux condensation or fractionation in the section below the first heavy gas oil product draw tray. It is henced combined with the fresh feed for reprocessing in the heater and coke drums.
The main function of the heater is to provide heat for the endothermic coking reaction and due to the high temperature requirement at the outlet condition of 920.degree. F. or thereabouts, the heated mass itself has undergone simultaneous vaporization and thermal decomposition in the heater tubes which can be compared to a visbreaker unit operation on account of its analogy with regards to temperature, residence time and %hydrocarbon conversion, which for example, are shown below:
______________________________________ Residence Heater Outlet % Conversion Type of Unit Time, min. Temp., .degree.F. (to light oil EP) ______________________________________ Visbreaker, 1.0 905. 14.4% coil type Visbreaker, 4.0 851. 14.4% soaker type Coker heater 0.5 920. 17.6% ______________________________________
By virtue of its high conversion the coker heater's liquid effluent is thermally unstable, asphaltenes are formed and well on its way to further polycyclic condensation to pitch formation. The vapors of approximately 28% of the total heater effluent mass consist of the same vapor components as the coke drum overhead up to the light gas oil components; plus, a minor amounts of the heavy gas oil and recycle stream cuts.
Pressure at the heater outlet is virtually controlled in the coke drum which in turn is monitored and controlled in the coker fractionator overhead receiver. This variable is hence limited and can not be liberally altered without affecting the operability of the process. The other controlled variable in this equipment is the heater outlet temperature which is in the usual manner is in the range of 900.degree. to 940.degree. F. Low temperature yields less hydrocarbons, produces a soft coke or pitch-like coke characterized by unacceptably high volatile carbonaceous matter content (VCM content), produces thermally unstable heavy hydrocarbons which eventually ends up in the heater tubes via the recycle stream (thus, promoting premature heater tube coke deposits) and the problematic tendency in the coke drums to foam more than the controllable amount. High temperature yields more hydrocarbons, produces a hard coke of low VCM content and the problematic tendency of the fluid, consisting mainly of the overheated unstable heavy petroleum residue to deposit coke in the tubewall which eventually builds up to a point that fluid flow is seriously impeded. Coke deposits tend to overheat the tubes which eventually become the overriding factor in shutting down the coker unit. Tubewall coke deposit however is routinely handled in a delayed coking process. With proper combination of fluid velocity, outlet temperature, crackability of feedstock, diluent addition and steam injection, the interval period between steam-air decoking well known to those skilled in the art can be at least one year. The most common cause of tubewall coke deposit normally observed in this heater can be traced to the laminar film of heavy crackable liquid coating the tubewall and aggravated by high film temperature due to very low heat film transfer coefficient. Such film is avoided by high fluid velocity and liquid dilution. Velocity is achieved by deliberately operating at no less than 5.0 feet per second inlet cold oil velocity (COV), but preferably more than 6.0 feet per second and typically augmented by steam injection. Dilution is provided by more recycled material and possibly the addition of non-crackable paraffinic lighter hydrocarbons in the fresh feed. Dilution also provides the opportunity for the minute size asphaltene particles to be released from the tubewall and become part of the fluid in the turbulent region. Steam injection on the other hand, by virtue of its lighter density, would tend to concentrate in the center of the tube for certain flow conditions and hence effecting a heavy liquid velocity much lower than the average fluid velocity. More vaporization effected by more steam injection would not help a severely coking tube with very low inlet COV. A unique case of premature tube coking is a very low heater outlet temperature which eventually produces a thermally unstable heavy hydrocarbon vapor in the coke drum and ends up in the heater tubes via the recycle stream.
At the heater outlet temperature typically above 920.degree. F. to as much as 940.degree. F., the heavy gas oil and recycle cuts, considered more refractory than the cracked virgin residue, would partially vaporize and leaving in the fluid more thermally unstable liquid residue. The dilution effect of such materials are diminished and hence the coking tendency in tubes. For a usual outlet pressure of 50 psig, paraffinic stocks are seldom observed at 950.degree. F. Aromatic stocks which are considered more refractory are sometimes operated at more than 950.degree. F.
To minimize coke deposit in the heater transfer line the effluent is transported rapidly to the coke drum within 5 to 10 seconds and a fluid velocity no less than 35 per second but preferably as close to 65 feet per second. Fluid temperature decreases 15.degree. to 25.degree. F. indicating reaction is actually taking place in such line. The highly unstable liquid portion decomposes to pitch-like carbonaceous material upon entry into the coke drum in approximately 20 minutes and the solid, spongy looking product coke, as is well known in the art, is finally produced after soaking for more than 3 hours.
The reaction mechanism in the coke drum is analogous to that in the heater which namely, thermal decomposition and subsequent combination of formed radicals by the the process of dehydrogenation. with declining temperature and readily available reactants in the liquid phase the radical bonding may be repeated extensively over a period as long as 24 hours forming coke characterized by its highly carbonaceous structure. The vapors produced in the heater are thermally stable and simply passes through the coke bed, uncracked, and hence mixes with the vapors produced in the coke drum itself.
The endothermic coking reactions, vaporization of liquid hydrocarbons and heat losses utilize the sensible heat available from the incoming heater effluent resulting to a substantial reduction in temperature. The vapors driven off the reacting liquid mass are at a temperature corresponding to their dew point temperature at a total pressure imposed by dynamic pressure losses and absolute pressure controlled on the fractionator overhead receiver. Every delayed coker unit with a given set of operating conditions and feedstock properties is observed to established a definite but limited relationship of heater outlet and coke drum vapor temperature in the range of 90.degree. to 135.degree. F. difference, more typically, 100.degree. F.
It is interesting to note that for the same heater outlet temperature, such difference in temperature decreases, or hence the coke drum overhead vapor temperature increases, with corresponding increase in the quantity of recycle material which clearly indicates an excess of heat has been made available in the total coke drum feed material. Such vapor is subsequently quenched in the overhead line to suppress further coke formation. In order to effectively avoid excessive and problematic coke deposit in the overhead line and valves, this quenched temperature is maintained at about 780.degree. to 800.degree. F. The quenched vapor material enters the fractionator where the recycle stream is produced by the process of condensation or fractionation in the tower section below the first heavy gas oil draw tray which also effectively cleanses off the coke carryover particles and controls the end boiling point of the heavy gas oil product. The total recycle material consisting of the condensed material produced by the process of quenching and fractionation or condensation combines with the fresh feed petroleum residue in the fractionator bottoms and hence charged to the heater. The quotient of the recycle of fresh feed is commonly called in the art as the "recycle ratio" or for convenience, the quotient of the heater charge to fresh feed on a volumetric basis is hereinafter called "total feed ratio" (TFR). For clarity, for example, a 0.10 recycle ratio is the equivalent of 1.10 TFR.
At 1.10 TFR the initial 5% boiling range of the recycle material is approximately 790.degree. F. to 830.degree. F. and about 690.degree. to 790.degree. F. for an equivalent 1.25 TFR. Such boiling range is vaporized in the heater outlet up to approximately 830.degree. F. cut point. Hence, at 1.10 TFR about 5% of the recycle material and about 10% at 1.25 TFR are the vaporized portion of the recycle in the heater outlet at the pressure prevalent in the art of approximately 50 psig.
The liquid portion of the recycle material enters the coke drum bottom, vaporizes further and therefrom undergoes thermal scission and extensive coking reactions along with the fresh feed material producing the gases, coke and hydrocarbons; plus, the freshly made recycle material with a boiling point range above the heavy gas oil. The quantity of the liquid portion of the recycle which actually undergoes thermal cracking in the coke drum is not measurable, but its effect on the coke yield is well known by those skilled in the art.
For a given coke drum pressure and heater outlet or coke drum vapor temperature, an increase in TFR increases the coke yield almost linearly up to a value of 1.30 TFR and hence becomes insignificant at 1.50 TFR and thereafter. The heavy gas oil returned to the heater as recycle, during very high recycle rates, is predominantly light gas oils that have been exposed many times to thermal cracking reactions and hence becomes insensitive to coking conditions and furthermore, said gas oils have a boiling range below 830.degree. F. which is readily vaporized in the heater outlet. The contribution of the recycle material to the coke yield can be as much as 7.0 to 11.0 weight% of the fresh feed at 2.0 TFR; 5.5 to 7.0% at 1.5 TFR; 4.0 to 5.0% at 1.25 TFR; 2.5 to 3.5% at 1.05 TFR; and, 0.0% TFR--the lower ranges of which are realized at lower pressure and higher temperature, typically below 25 psig and above 920.degree. F. heater outlet temperature.
For the same recycle ratio and temperature, coke yield decreases with decreasing pressure since more hydrocarbons are vaporized at lower operating pressure. Operation at lower pressure however would lower vapor density in the coke drum as well as in the fractionator, necessitating an increase in diameter of said equipment if the same velocity is maintained. Moreover, the fractionator overhead compressor capacity would have to be increased for the corresponding decrease in inlet pressure. The choice therefore for the design pressure is purely the economical viability of operating at lower pressure and increased hydrocarbon yield for more investment in additional equipment capacity. The choice in pressure above the coke bed is usually between 10 to 50 psig for the case of making fuel grade coke and as much as 100 psig for the case of making high grade preminum coke; more typically, 12 to 35 psig for the former case and 50 to 75 psig for the latter case, with a corresponding decrease in coke yield of approximately 1.0 to 2.0%--the higher range of which are realized at lower recycle and higher heater outlet temperature.
For the same recycle ratio and drum pressure, coke yield decreases with increasing furnace outlet temperature since more hydrocarbons are vaporized and hence unavailable for further coking reaction. The furnace temperature is constrained over a narrow range by several operating variables. On the lowest end of the range, this temperature is determined by uncontrollable foaming in the coke drum or highly unstable heavy hydrocarbons which tend to prematurely deposit coke in the heater tubes or it produces soft coke or pitch-like material with unacceptably high VCM content. On the highest end of the range, this temperature is determined by low VCM content commonly associated with the coke hardness or the unacceptably longer hydraulic jet decoking period required to remove coke inside the cold drum or the coking tendency inside the heater tube due to overcracking and diminished dilution due to over vaporization of the fluid at the heater outlet. The choice of temperature is determined during operation when all the interaction of all variables are known for the charge stock on hand. For more aromatic stocks, the range is higher at about 935.degree. F. to 960.degree. F. and lower for paraffinic stocks, typically at 910.degree. to 940.degree. F. The high range are seldom exceeded due to the aforementioned constraints, but more particularly due to excessive heater tube coke deposit and very hard coke in the drum. A 30.degree. F. rise in temperature corresponds to approximately 2.0 to 4.0% decrease in coke yield--the higher range of which are realized at lower recycle ratio and lower pressure.
Stocks of high boiling virgin or cracked petroleum residue are used for coking operations which may be suitable for blending to heavy fuel oil such as fuel oil no. 6. Delayed coking, along with other heavy petroleum residue processes reduce heavy fuel oil inventory and are normally justified since the price of fuel oil is substantially lower than the combined prices of the more valuable products. Pricing of fuel oil however is erratic and during times of market upswing, this price differential has diminished to a point that some existing cokers in the current state of the art can not operate at a profit and moreover, a new unit may not be justified economically. But with the advent of much heavier future crude stocks of higher metals, sulfur and other impurities of varying and unpredictable properties, delayed coking is still competitive. With the possibility of crowded market for fuel grade coke, the demand for an improved delayed coking process featuring a substantial decrease of coke yield and enhanced hydrocarbon yield has never been as pronounced. In addition, due to changing petroluem products market demand, supply and prices, not to mention the poor quality of crude stocks, it is almost imperative to have a delayed coking process with plenty of operating and design flexibilities. This invention accomplishes these objectives and also compliments other innovations for hydrocarbon yield enhancement.
The process of introducing an interim drum between the heater and the coke drums may be compared to another thermal craking process called "visbreaking". In one configuration of this process, a so called "soaker drum" between the heater and the fractionator is introduced to operate the heater at lower temperature than the conventional "coil" type visbreaking process, without an interim drum. The same conversion and slightly different product distribution are accomplished in the soaker type process by careful manipulation of residence time and heater outlet temperature. It should be noted that visbreaking is quite analogous to delayed coking without the coke drums. The former's heater effluent is its product, while the latter's heater effluent is fed to the coke drum for further thermal treatment.
Said interim drum in this invention is hereinafter called the "flasher drum". In its basic configuration shown in FIG. 1, the heater effluent vapor is separated from the liquid portion used as the only feed to the coke drum. Such vapor, present in the coke drum in the prior art, exerts a partial pressure which suppresses vaporization of liquid hydrocarbons. The absence of such vapor in the coke drum in this invention induces more hydrocarbons to vaporize and hence avoid further thermal coking reaction leading to enhanced hydrocarbon yield and minimum coke make. The introduction of the flasher drum also affords higher heater outlet temperature at high recycle ratio, accommodate further addition of heat and since the vapor portion of the recycle is separated from the flasher drum, the liquid portion of the recycle fed with the fresh feed into the coke drum is of consistently higher boiling point range corresponding to a lower TFR than the original TFR charged into the heater. Hence the significant effect of high TFR to coke yield is minimized. The introduction of heated gases and more particularly, hydrogen gases at varying rates into the flasher drum affords further flexibility in this invention. This process, commonly referred to as hydropyrolysis, is the reaction of hydrogen gas with the thermally cracked hydrocarbon radicals in the absence of a catalyst making a very stable hydrocarbon compound incapable of further bimolecular reaction with another radical. This reaction is the reverse of coking reaction and hence minimizes coke production.
Hydropyrolysis have been investigated solely for the purpose of competing directly with delayed coking process and none has fully appreciated the possibility of intermingling its good points for the enhancement of hydrocarbon yield and substantial coke yield reduction. For example, Bunger, et al., (ACS Symposium Series 163, 1981, pp. 369-380) disclosed in his research at the University of Utah that complete hyropyrolysis of heavy crudes without the formation of coke is possible in a reactor-heater coil at high temperature, high hydrogen partial pressure and high hydrogen consumption with as little as one minute residence time.
A mild form of hydropyrolsis is also disclosed in Hayashi's U.S. Pat. Np. 4,132,742 to refine or remove problematic impurities in the petroleum residue.
The common denominator and basic principle underlying this invention and that the prior art's yield enhancement techniques is the vaporization of liquid hydrocarbons in the coking mass, whereby, further polymerization of hydrocarbon radicals is effectively minimized at varying degrees. This is also true in most recent inventions accomplishing the same objective which is also compatible with this invention as well as the prior art. For example, U.S. Pat. No. 4,358,366 discloses the injection of hydrocracking catalyst and hydrogen gas to coke drum improves hydrocarbon yield and reduces coke make. U.S. Pat. No. 4,378,288 claim increase coker distillate yield by adding a small amount of a free radical inhibitor consisting of hydroquinone and N-phenyl-2-naphylamine to the coker feed material. Similarly, U.S. Pat. No. 4,399,024 teach the promotion of rapid liquid hydrocarbon vaporization by addition of one of the prescribed additives, consisting of metal salts of dialkyxanthogenic acids and others. The reaction explained being the stabization of free radicals formed during the thermal scission process typically observed in delayed coking.